Quantity Discounts are allowed based on the volume / value of purchases made by the customer for a particular period. The discount is allowed at the end of a particular period based on the pre-agreed rates entered into between the supplier and the recipient. Such discounts will be eligible for exclusions by way of credit notes, only where the supplier is in a position to link the discount to each and every invoice, and the recipient reverses the credit to the extent of such discount.
(ICAI FAQ PUBLICATION 06-09-2017 VALUE OF TAXABLE SUPPLY: FAQ NO. 54)